Ever commit to repo and then belatedly realize that the authorship metadata isn't what it should be? Easy to do when pair programming.
Here's a nifty little script for fixing authorship of commits within a Git repository. It uses a Git plumbing command called filter-branch.
Consider the GitHub-provided script a starting point for your own, rather than a copy/paste solution. Read filter-branch's documentation carefully before using. filter-branch changes the SHA-1 value of any commits affected, which means that every descendant commit's SHA-1 will also be re-written. Of course, there are also the usual caveats about the pitfalls of changing history on a shared repo.
More info: rewriting history from Scott Chacon, in Pro Git.